Systems for projecting images onto screens are known in the art. Systems based on incandescent or arc lamps are often used in projection displays. However, such sources have limited lifetimes and poor output stability over the life of the light source. In addition, spectral components, such as UV and IR, that are not needed for display purposes must be filtered out of the emissions to avoid damage to other components or the viewer's eyes.
Sources based on combinations of Light Emitting Diodes (LEDs) or semiconductor lasers that emit red, green and blue light can be used as a solid state light source. Since LED and laser emissions are typically spectrally narrow band light sources, the use of color filters for the display can be omitted. Also the optical output of semiconductor lasers and LEDs can typically be electrically modulated at a rate that is sufficient for a time sequential color display. Furthermore most solid state lasers emit linear polarized light which further improves the efficiency of light production and simplifies the optics in projection systems.
Unfortunately, individual solid state light sources do not provide sufficient brightness for many projection display applications; hence, arrays of LEDs must be utilized to obtain sufficient output. A significant fraction of the light generated in an LED array is lost. Conventional LEDs emit light through the top, the bottom and the side facets. Most high power red, green and blue LEDs contain an optically transparent substrate. If the LEDs are placed on a planar heat sink, the substrate and top surface of the LED act as an optical waveguide, guiding the light between neighboring LEDs. This waveguide effect transports a significant fraction of the light emitted through the side facets of the LEDs to the outer edge of the array. The light is attenuated during this transportation process and emitted at a place where it is only partially captured by the collimating optics.
Broadly, it is the object of the present invention to provide an improved solid state light source.
It is further object of the present invention to provide an improved solid state light source that more efficiently captures the light leaving the side facets of the LEDs than prior art light sources.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.